Glycogen storage disease (GSD) type 1 is an autosomal recessive disease caused by the deficiency of glucose-6-phosphatase (G6Pase). Diagnosis of GSD type 1a currently is established by demonstrating the lack of G6Pase activity in the patient's biopsied liver specimen. Our cloning of the G6Pase gene and identification of mutations within the gene that cause GSD type 1a allow for the development of a DNA-based diagnostic method. We have characterized the G6Pase gene of 70 unrelated GSD type 1a patients and detected mutations in all except 17 alleles (88%). Sixteen disease-causing mutations were uncovered and the prevalent mutations in Caucasians, Hispanics, and Chinese were identified. This DNA-based diagnosis now permits prenatal diagnosis among at-risk patients and serves as a data-base in screening and counseling patients clinically suspected of having this disease. We have characterized mutant G6Pases generated by near-saturation mutagenesis of codons 83, 222, and 295 known to be important for G6Pase catalysis. We showed that Arg-83 may be involved in stabilizing the enzyme-phosphate intermediate formed during G6Pase catalysis and Arg-295 may be involved in salt-bridge formation. Mutagenesis studies also show that His-119 is the phosphate acceptor in G6Pase catalysis. A null G6Pase mutation in embryonic stem cells has been generated by targeted-disruption of the G6Pase gene. These genetically modified cells were used to establish G6Pase-deficient mutant mice for use as a model of GSD type 1a. Our studies show that the G6Pase-deficient mice mimic GSD type 1a in humans. Methionine adenosyltransferase (MAT) is a key enzyme in transmethylation and transsulfuration. Alpha/beta-MAT deficiency causes isolated persistent hypermethioninemia, unusual breath odor or neural demyelination. We have characterized the human alpha/beta-MAT gene, identified several mutations in the gene of MAT deficient patients, and showed that these mutations partially inactivate MAT activity. These results establish the molecular basis of this disorder and allow for the development of DNA-based methods to diagnose hypermethioninemic individuals suspected of having abnormalities at this locus. The pregnancy-specific glycoproteins (PSGs) form a large family of essential pregnancy proteins, but their biological function is unknown. We show that the PSG11s receptors are present on cells of the myeloid cell lineage, suggesting that one of function of PSG is to interact with cells of the maternal immune system.